1. Field of the Invention
The invention relates to a display device for displaying gray tone images with a monitor driven by s electron rays.
2. Description of the Related Art
Images used, for example, for diagnostic purposes are usually displayed as gray tone images in medical technology. The smallest details are important in the display of these medical images. The object is accordingly to achieve an optimized display with a large number of brightness levels which are distinguishable by the human eye.
WO 96/19899-A3 describes a monochromatic monitor which is driven with three electron guns (triple gun). These three electron rays are imaged in a single point. The image information with which these electron rays are fed, however, is always the same. The generation of a pixel by three electron rays achieves a high brightness and a high sharpness.
Monitors have a representation curve which differs from the optimum perceptional representation curve. Typical monitor curves show an insufficient resolution of the individual gray tones in the low or darker gray tone region. The insufficient resolution of the gray tones in this region means that different gray tones cannot be distinguished or that several different gray tones are displayed as only one gray tone value. To render these differences in gray tones visible or to correct them, a correction in the brightness distribution of the monitor representation curve is to be carried out.
In prior-art correction arrangements, the same number of gray tone values is assigned to a corresponding number of gray tone values. The assignment instruction for correcting the brightness distribution of the monitor, or the representation curve thereof, thus assigns the same gray tone values of the output video signal to several gray tone values of an input video signal, which leads to a loss in displayable gray tone values.
It is accordingly an object of the invention to provide a simple device for adapting the brightness distribution of the monitor to the optimum perceptional brightness distribution.
According to the invention, this object is achieved in that a correction unit is provided for generating s output image signals As (i.e., s greater than 1), which comprise gray tone values forming part of a set K of correction gray tones, from an image signal which contains gray tone values forming part of a set M of original gray tones, and in that said s output image signals As, after a D/A conversion, are designed to be supplied each to an electron ray of the monitor for a brightness-corrected display of a gray tone in a point of the monitor.
Each monitor has an individual brightness distribution where the problems mentioned above in the representation of an image signal occur. It is accordingly necessary to adapt the image signal with its coded image information to the brightness distribution of the monitor so as to achieve a perceptionally optimized rendering. The image signal comprises, for example, a coded medical image in its original recorded form, which is supplied to the correction unit. The correction unit receives this image signal, which contains gray tone values from a set M of original gray tones. This set M of the original gray tones is associated with the set K of the correction gray tones in dependence on the brightness distribution of the monitor. This leads to the generation of s digital output image signals which are each subjected to a digital/analog conversion.
The monitor is driven by a number s of electron rays, s being a given natural number. The s analog output image signals are each supplied to one of the s electron rays which drive the monitor. The s electron rays are imaged in one point of the monitor, so that a mixing of several gray tone values takes place here. The original gray tone, which does not correspond to the perceptionally optimized representation, is displayed with brightness correction owing to this mixing of the s electron rays in one point.
In this arrangement according to the invention, input gray tones are assigned to several output gray tones in a correction unit. Each of the several output gray tones is supplied to an electron ray for displaying the output gray tones on the monitor. The several electron rays are imaged in one point of the monitor, so that a mixing of the several output gray tones is achieved. Thus a different gray tone is mixed for each input gray tone from several output gray tones, and a perceptionally optimized display of the image signal on the monitor is created.
The n-bit wide image signal is capable of containing 2n different gray tone values. These original gray tones are assigned to s correction sets of 2n gray tone values each in the correction unit. The number of the correction sets is determined here by the number s of electron rays. Each correction set, for example, comprises 2n different correction gray tones. Each original gray tone is associated with a gray tone value from each correction set, so that s output image signals are generated. The assignment of s correction gray tone values to each original gray tone value achieves the adaptation of the brightness distribution of the monitor to the optimum perceptional brightness distribution.
The number of possible gray tones which can be displayed is increased through the assignment of several correction gray tones.
In a further embodiment of the invention it is found to be advantageous to arrange a photosensor in the vicinity of the monitor, which sensor influences the correction unit in dependence on the ambient light. The use of such a photosensor involves the use of several correction tables for different ambient brightness values. Depending on the value measured by the ambient light sensor, the corresponding correction table is then selected during operation of the device.
To determine the brightness distribution of the monitor, calibration measurements are carried out with a luminance sensor. The luminance curves are measured individually for each electron ray and collectively for all electron rays. The luminance sensor measures the brightness of the pixels directly on the surface of the picture tube of the monitor. The luminance curves thus determined are supplied to the correction unit for calculating the correction tables.
In a further embodiment of the invention, it is advantageous to use a photosensitive sensor first for determining the luminance curves. After the luminance curves have been laid down, this sensor is utilized as an ambient light sensor through a change in its orientation.
In an alternative embodiment of the invention, the luminance curves of the monitor are synthetically generated through a parameter description. Since the brightness distribution of the monitor also depends on user-specific settings, it is very cumbersome to measure all possible settings in advance and then to associate the original gray tone values with the correction gray tone values in accordance therewith. The luminance curves can be approximated with a very good degree of correspondence by a simple parameter description through the creation of a calculation instruction in which the luminance values depend on the brightness and contrast values. The brightness distributions or luminance curves are then calculated, and subsequently the assignation of the original gray tone values to the correction gray tones is carried out in the look-up table so as to set a display mode which is perceptionally optimized.
In a further embodiment of the invention, it is found to be advantageous in a triple-gun monitor, which is driven by three electron rays, to assign three correction sets of gray tones to the set of original gray tones of the image signal in the correction unit for generating three output image signals. One gray tone value from each of the three correction sets is then assigned to each gray tone value from the original gray tones. These three output image signals, after a digital/analog conversion, are each supplied to a respective electron ray of the triple-gun monitor and are imaged in a common point on the monitor with a corrected gray tone value for each original gray tone value to be pictured.
It is an advantage of this invention that three-channel graphic cards for color monitors can be used. These three-channel graphic cards are complemented with a suitable correction unit for adapting the brightness distribution of the monitor to the optimum perceptional brightness distribution. This means that no new hardware need be developed.
In a further embodiment of the invention, it is advantageous that the n-bit wide image signal is supplied to s correction sets Ks, in the correction unit, which sets have a bit depth smaller than n. The assignment of s correction sets with corresponding gray tone values renders it possible to provide the correction sets with fewer gray tone values than the original number of gray tones, thanks to the subsequent mixing of the electron rays. When the three output image signals As are mixed, a theoretical number of gray tone values is possible corresponding to the permutation of the three gray tones of the correction sets, so that the quantity of the correction gray tones can be smaller than the quantity of the original gray tones. The digital/analog conversion may accordingly be designed to have a processing width smaller than n as well.
Such gray tone images are recorded by X-ray devices, computer or magnetic resonance tomography devices, or ultrasound devices.